It is known in the art to use microwave heating to promote the progress of one or more sample preparation steps or of one or more chemical synthesis steps. For example, microwave heating of chemical reactions in sealed containers to promote an organic reaction was described by Giguere et al., in "The use of microwave ovens for rapid organic synthesis", Tetrahedron Letters 27: 279 (1986). Early studies showed that microwave heating can promote the progress of a chemical reaction and offers a dramatic decrease in reaction time. Those findings stimulated the study of microwave heating, and advances in the field are described in several reviews (such as in Abramovitch, R. A., "Applications Of Microwave Energy in Organic Chemistry, A Review", Organic Preparations Procedures International 23:683-711.)
These developments in microwave-assisted synthesis now offer fast synthesis procedures while offering better yields and improved selectivity. Developments in microwave-assisted synthesis and extraction techniques aid in the generation and extraction of complex reaction mixtures. Microwave-assisted synthesis allows fast product generation in high yield under uniform conditions, and is considered to be well suited for performing combinatorial chemistry.
The conventional microwave-assisted heating apparatus typically employs a microwave energy source, usually a magnetron or traveling wave tube source, coupled to a waveguide which serves to guide microwave energy in a forward direction to a container that contains a sample that is to be subjected to microwave-assisted heating.
It is important to monitor the progress of a chemical reaction in a microwave assisted heating system. Without some means for determining the progress of the reaction, the operator may discontinue the progress of the reaction prior to its natural or desired point of completion, or may allow the reaction to progress beyond the desired point whereupon unwanted byproducts or degradation may occur. Typical means for monitoring include thermocouples, temperature probes, pressure probes, and similarly invasive devices, which may not be compatible with an applied microwave field. Proper monitoring of the chemical reaction allows the operator to optimize the time spent on the reaction and improve the overall synthetic yield. Instruments are typically employed for monitoring the state of the chemical reaction by observing the temperature, color, pH, viscosity, and the like of the material subject to the reaction. However, it would be desirable and of considerable advantage to provide an improved, non-invasive system for monitoring the progress of a chemical reaction and a microwave-assisted heating system that offers a more accurate indication of the progress of the chemical reaction.